Cannot use PASV when EPSV was used before.

[wasabii]

I'm trying to figure out a way to solve this. It seems to be a limit in ProFTPd, that you can't switch from EPSV to PASV. However, I have a client library that tries to use EPSV, and if that fails, switches to PASV on it's own and tries that instead.

Why would changes to this be prevented? Just a limitation?

[Castaglia]

What're the error/logs that you see, when trying to switch from EPSV to PASV? I can't recall any particular limitations that you should be encountering...

[wasabii]

So, found the error. I was using mod_proxy. The error comes from my backend. I forced ProFTPd's mod_proxy to always use PASV, regardless what the client wants, and it's fine now.

[Castaglia]

Glad to hear that you found the cause and the fix. Would you be willing to provide some example logs, from the client- and server-side of things, so that I can add those logs as examples, in docs, of symptoms other users might see, and how to address them?

[wasabii]

Unfortunately I solved it a week ago, and lost the logs. ;(

The backend server I'm using doesn't support EPSV. It's a .NET library called FubarDev.FtpServer And the client involved was FluentFTP, a .NET FTP client library.

This library, in default operation, attempts to open an EPSV connection, and then when that fails, attempts to move down the list trying different modes.

ProFTPD was passing this all back to the backend. The backend was rejecting the EPSV. And then when the client next tried PASV, it was throwing an error about attempting to switch.

So, to shield all that, I've forced ProFTPd's mod_proxy to always use PASV, through ProxyDataTransferPolicy. This allows the client to connect with whatever it wants (it picks EPSV) but always do PASV to the server. So the server doesn't know any better.

[Castaglia]

OK, thanks for the info! I'll see if there's a way to make mod_proxy better detect/handle cases where the backend FTP server does not support EPSV, but the frontend client issues an EPSV (or EPRT) command.

Filed this for tracking as Castaglia/proftpd-mod_proxy#146

[wasabii]

Oh wow! Thanks! It took me a couple days to work this out, so that might save others the trouble.

You rock.